In mobile radio communication networks and particularly in digital networks (such as GSM) the quality of transmission can vary within a wide range depending on the location (distance to the next base station), the travelling speed and the environment (buildings, mountains etc.) of the mobile station at a certain instant. It is important for the operator of such networks to know where the reception quality is unsatisfactory in order to be able to improve the network by appropriate measures.
So far there are known two types of methods for checking the transmission quality of mobile telephone systems: One can be qualified as "manual" method and the other as "automatic" method. In the "manual" method an expert is driving a car through the area to be assessed while listening e. g. to the news transmitted via the mobile communication system. The expert is continuously writing down his opinion on the quality of reception. E. g. he classifies the reception quality--on the basis of his experience--into different rating classes such as "good", "acceptable" and "bad". It is evident that the results of this method are not reproducible. Various experts have different assessments even under similar conditions. Even if the same expert drives along the same route at different times, the results may differ and it is not possible to determine, whether the difference was just in the assessment or whether it was in the actual transmission quality.
The "automatic" method is based on the fact that e. g. the bit error rate of digital transmissions is a known and measurable quantity. The error rate is therefore used for rating the transmission quality. Tests have shown that the bit error rate and the audible transmission quality (in the sense of the intelligibility of a transmitted sentence) often do not correlate: The bit error rate may be acceptable or good, whereas the intelligibility is bad and vice versa. The bit error rate assessment is only viable for a digital transmission system. An analog speech transmission cannot be rated by this method.
There are other automatic methods for determining the transmission quality of a channel. GB-1 455 712 e. g. teaches comparing the input and the output signal of the system and determining some sort of quality according to the signal difference. A definition of the quality is not disclosed. The prior art document EP-0 114 463 teaches transmitting a plurality of tone signals, digitizing the received signal and determining the Fourier transform of the tone signals in order to determine the transmission quality.
The problem of the known techniques for rating the transmission quality is, that the basical correlation between the transmission quality of a specific physical or software parameter (tone signal, error signal, bit error rate etc.) and the rating done by a test person is unsatisfactory.